Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies Stems Chapter 6 Copyright © McGraw-Hill Companies Permission.

Chapter 6

Outline

• External Form of a Woody Twig• Stem Origin and Development• Stem Tissue Patterns• Herbaceous Dicotyledonous Stems• Woody Dicotyledonous Stems• Monocotyledonous Stems• Specialized Stems• Wood and Its Uses

External Form of A Woody Twig

• Woody twig consists of an axis with attached leaves. Alternately or Oppositely arranged.

- Leaves attached at a node. Stem region between nodes is an

internode. Leaf has a flattened blade and is

usually attached to the twig by a petiole.

External Form of A Woody Twig

• Axil - Angle between a petiole and the stem. Axillary Bud located in axil.

- Terminal Bud often found at twig tip.• Stipules - Paired appendages at the base of a

leaf. Often remain throughout leaf life span. Deciduous trees and shrubs have dormant

axillary buds with leaf scars left after leaves fall.

- Bundle scars mark food and water conducting tissue.

Origin and Development of Stems

• Apical meristem is dormant before the beginning of the growing season. Protected by bud scales and by primordia.

• When a bud begins to expand, apical meristem cells undergo mitosis and three primary meristems develop.

Primary Meristems

Protoderm - Gives rise to epidermis. Procambium - Produces primary xylem

and primary phloem cells. Ground Meristem - Produces tissues

composed of parenchyma cells.- Pith- Cortex

Origin and Development of Stems

• Narrow band of cells between the primary xylem and primary phloem may become vascular cambium. Cells produced by the vascular cambium

become components of secondary xylem and secondary phloem.

• In many plants, a second cambium, cork cambium, arises. Produces cork cells and phelloderm cells.

Tissue Patterns in Stems

• Steles Stele is made up of primary xylem, primary

phloem, and pith (if present).- Protostele - Solid core.- Sphinosteles - Tubular with pith in center.- Eusteles - Vascular bundles.

Tissue Patterns in Stems

• Dicotyledons - Flowering plants that develop from seeds having two seed leaves.

• Monocotyledons - Flowering plants that develop from seeds with a single seed leaf.

Herbaceous Dicotyledonous Stems

• In general, annuals are green, herbaceous plants. Most monocots are annuals, but many

dicots are also annuals.• Herbaceous dicots have discrete vascular

bundles composed of patches of xylem and phloem. Procambium produces only primary xylem

and phloem, but vascular cambium arises later and adds secondary phloem and xylem to the vascular bundles.

Woody Dicotyledonous Stems

• Vascular cambium of a typical broadleaf tree produces relatively large vessel elements of secondary xylem (spring wood). Xylem produced next has smaller or fewer

elements, and is referred to as summer wood.

One year’s growth of xylem is called an annual ring.

• Vascular cambium produces more secondary xylem than phloem, thus bulk of a tree trunk consists of annual rings of wood. Examining rings can determine the age of

a tree, and provide some indications of climatic conditions.

• Vascular Rays consist of parenchyma cells that function in lateral conductions of nutrients and water. Xylem Ray Phloem Ray

• Older, darker wood at the center is called heartwood, while the lighter, still-functioning xylem closest to the cambium is called sapwood. Formed at roughly the same rate as

heartwood.- Softwood - Xylem consists primarily of

tracheids; no fibers of vessel elements. Cone-bearing trees.

• Bark - Refers to all the tissues outside the cambium, including the phloem. Mature bark may consist of alternating

layers of crushed phloem and cork.

Monocotyledonous Stems

• The stems of most monocots have neither a vascular cambium nor a cork cambium and thus produce no secondary vascular tissues or cork. Xylem and phloem exist in discrete

vascular bundles.- Secondary meristem produces only

parenchyma cells to the outside and secondary vascular bundles to the inside.

Specialized Stems

• Rhizomes - Horizontal stems that grow below-ground.

• Runners - Horizontal stems that generally grow along surface.

• Stolons - Produced beneath the surface of the ground and tend to grow in different directions.

Specialized Stems

• Bulbs - Large buds surrounded by numerous fleshy leaves, with a small stem at the lower end.

• Corms - Resemble bulbs, but composed almost entirely of stem tissue.

• Cladophylls - Flattened, leaf-life stems.• Thorns• Tendrils

Wood and its Uses

• In a living tree, 50% of the wood weight comes from water content. Dry weight is composed of 60-75% cellulose

and 15-25% lignin.• Density and Durability are two of the most

important characteristics in commercial wood.

Wood and its Uses

• Sawing Radially cut (quartersawed) boards show

the annual rings in a side view. Tangentially cut (plain-sawed) boards show

annual rings as irregular bands of light and dark streaks.

Wood and its Uses

• Knots Bases of lost branches covered by new

annual rings produced by the cambium of the trunk.

Found in greater concentration in older parts of the log, towards the center.

Wood and its Uses

• Wood Products About half of US and Canadian wood

production is used as lumber, primarily for construction.

- Veneer - Thin sheet of desirable wood glued to cheaper lumber.

Second most extensive use of wood is pulp. In developing countries, approximately half

of cut timber is used for fuel.- Less than 10% in US and Canada.

Review

• External Form of a Woody Twig• Stem Origin and Development• Stem Tissue Patterns• Herbaceous Dicotyledonous Stems• Woody Dicotyledonous Stems• Monocotyledonous Stems• Specialized Stems• Wood and Its Uses

Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies Stems Chapter 6 Copyright © McGraw-Hill Companies Permission.

mcgraw hill companies

primary phloem cells

primary xylem

uses slide

display slide

woody twig woody twig

pith cortex slide

woody twig stem origin

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